Quick Answer
The answer is the -f (fragment) option, along with -D (decoy) and --mtu (custom MTU), as these three Nmap evasion techniques are specifically designed to bypass IDS/IPS detection. The -f flag works by splitting scan packets into tiny 8-byte fragments, which many intrusion detection systems struggle to reassemble in real time, allowing the scan to slip past signature-based filters. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of how attackers manipulate packet structure to evade network defenses, often appearing in scenario-based questions where you must select the correct evasion method from a list of Nmap options. A common trap is confusing -f with --data-length, which pads packets rather than fragments them. To remember the core evasion trio, think “Frag, Decoy, and MTU” — the three tools that hide your scan from prying eyes.
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which THREE of the following Nmap options can be used to evade detection by IDS/IPS? (Select three)
-sV (version detection)
Why wrong: Version detection is not an evasion technique.
-f (fragment packets)
Fragmentation evades packet inspection.
-sI (idle scan)
Idle scan bounces off a zombie host.
-D (decoy scan)
Decoy scan uses spoofed IPs to confuse IDS.
-O (OS detection)
Why wrong: OS detection sends probes that may be detected.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
-f (fragment packets)
Option B is correct because the -f flag fragments the crafted packets into smaller pieces, typically 8-byte fragments, which can bypass simple IDS/IPS signatures that expect full unfragmented packets. This technique exploits the fact that many intrusion detection systems struggle to reassemble fragmented packets in real time, allowing the scan to evade detection.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
-sV (version detection)
Why it's wrong here
Version detection is not an evasion technique.
-f (fragment packets)
Why this is correct
Fragmentation evades packet inspection.
Related concept
Read the scenario before looking for a memorised answer.
-sI (idle scan)
Why this is correct
Idle scan bounces off a zombie host.
Related concept
Read the scenario before looking for a memorised answer.
-D (decoy scan)
-O (OS detection)
Why it's wrong here
OS detection sends probes that may be detected.
Common exam traps
The trap here is that candidates often think version detection (-sV) or OS detection (-O) are stealthy because they provide detailed information, but they actually generate more traffic and distinctive patterns that IDS/IPS easily flag.
Detailed technical explanation
Fragmenting packets works by splitting the TCP header across multiple IP fragments, which can confuse IDS/IPS that do not perform proper IP reassembly before inspection. However, some modern IDS/IPS (e.g., Snort with frag3 preprocessor) can reassemble fragments and still detect the scan. The -f flag in Nmap defaults to an 8-byte fragment size, but the --mtu option allows custom sizes to further evade detection.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
The correct answer is: -f (fragment packets) — Option B is correct because the -f flag fragments the crafted packets into smaller pieces, typically 8-byte fragments, which can bypass simple IDS/IPS signatures that expect full unfragmented packets. This technique exploits the fact that many intrusion detection systems struggle to reassemble fragmented packets in real time, allowing the scan to evade detection.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This CEH practice question is part of Courseiva's free EC-Council certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CEH exam.
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